The present invention relates to a crystal pulling apparatus for growing a rod-like semiconductor single crystal and pulling it from a crucible containing a melt.
When a semiconductor single crystal is grown from a melt in a single crucible, by means of the conventional Czochralski technique (to be referred to as the CZ technique hereinafter), impurity concentration distribution C in the longitudinal direction of the grown single crystal is represented by the following equation, as is well known: EQU C=kC.sub.0 (1-G).sup.k-1
where k is the segregation coefficient, C.sub.0 is the initial impurity concentration of the melt, and G is the solidification rate.
Therefore, the impurity concentration distribution in the longitudinal direction of the grown single crystal varies considerably, especially when segregation coefficient k is small, thereby significantly decreasing the formation yield of the single crystal having an impurity concentration within a predetermined range.
Conventionally, a pulling technique known as a double-crucible technique has been proposed, with the aim of solving the above problem and applied to single crystal growth of germanium or silicon. In this technique, two crucibles, i.e., an outer crucible and an inner crucible which communicates with the outer crucible, are used, and a melt surface in the inner crucible is maintained constant.
This conventional double-crucible technique for maintaining the melt surface constant is described in the Journal of Applied Physics Vol. No. 8, Aug. '58 "Floating Crucible Technique for Growing Uniformly Doped Crystals", W. F. Leverton, Author. In this technique, an inner crucible having a through hole at its bottom is placed in an outer crucible separated therefrom with a gap between it and the outer crucible. Therefore, constant the buoyancy and gravity of the inner crucible are balanced, and the surface of the melt in the inner crucible is maintained.
In addition, according to a technique described in Japanese Patent Publication No. 60-18634, an inner crucible is placed in an outer crucible and is moved downward therein by an inner crucible fixing support rod, or else the inner crucible is fixed at a constant position and the outer crucible is moved upward during a crystal pulling process. Thus, the height of the melt surface in the inner crucible is maintained constant during the crystal pulling process, while the melt in the outer crucible is supplied via a through hole in the bottom of the inner crucible, whereby a single crystal rod is pulled up from the inner crucible.
The above conventional double-crucible technique for maintaining the melt surface in the inner crucible at a constant level has the following advantage:
Assume that the impurity concentration of the melt in the outer crucible is C.sub.0, and that of the melt in the inner crucible is C.sub.0 /k (k is the segregation coefficient of the impurity).
The impurity concentration in a grown pulled single crystal becomes C.sub.0, and a melt, the amount and impurity concentration of which are equal to those of a melt portion used for crystal growth, is supplied continuously from the outer crucible to the inner crucible via the through hole. As a result, the impurity concentration of the melt in the inner crucible is maintained at constant value C.sub.0 /k. Therefore, the impurity concentration of the pulled single crystal is also maintained at constant value C.sub.0.
However, the conventional double-crucible technique for maintaining the level of the melt surface in the inner crucible does have the following drawbacks:
(1) When the diameter of the through hole in the bottom of the inner crucible is too large, the melt is exchanged between the outer and inner crucibles by means of convection, a vortex, and the like. As a result, the predetermined differing impurity concentrations in the inner and outer crucibles become undesirably averaged.
(2) Even when the diameter of the through hole is so small as to prevent exchange of the respective melts, impurity diffusion occurs via the through hole if the time required for melting a raw material, consisting of a group of polycrystals for forming a single crystal, is long, or if the time required for neckdown, for obtaining a smaller crystal diameter and increasing the crystal growth speed to remove dislocations, is long. As a result, it is difficult to maintain the impurity concentrations of the respective melts in the inner and outer crucibles at their predetermined constant values.
(3) After solidification of the melt progresses, and the bottom of the inner crucible comes into contact with that of the outer crucible, on account of a decrease in the amount of melt in the outer crucible, the impurity concentration in the pulled single crystal varies along with the solidification rate. In other words, a single crystal with a constant impurity concentration can be obtained only when solidification rate G falls within a range represented as follows: EQU 0.ltoreq.G.ltoreq.1-(h/H)
where H is the liquid surface height of the melt in the outer crucible when pulling is started, and h is the liquid surface height of the melt in the inner crucible which should be maintained at a constant value.